Method and apparatus of retrieval of coded information from symbols having coded inks having photoluminescent components with short and long time constants of decay after short wave illumination



F. HALVERSON METHOD AND APPARATUS OF RETRIEVAL OF CODED INFORMATION FROM SYMBOLS HAVING CODED INKS HAVING PHOTOLUMINESCENT COMPONENTS WITH SHORT AND LONG TIME CONSTANTS OF DECAY AFTER'SHORT WAVE ILLUMINATION Filed Feb. 9, 1966 V(AC, NO. I) V(AC,N0.2} -lv (0c,/v0. 1 woe, N0 2/ 3 Sheets-Sheet 1 INVENTOR. FREDERICK HALVE'RSON WWW A7 TORNf) NOV. 19, 1968 HALVERSON 3,412,245

METHOD AND APPARATUS OF RETRIEVAL OF CODED INFORMATI N FROM ,sYMEoLs HAVING CODED INKS HAVING PHOTOLUMINESCENT COMPONENTS WITH SHORT AND LONG TIME CONSTANTS OF DECAY AFTER SHORT WAVE ILLUMINATION Filed Feb. 9, 1966 5 Sheets-Sheet 2 IN VENTOR. FREDERICK HA L VERSON wmhm ATTORNEY NOV. 19, 1968 HALVERSON 3,412,245

METHOD AND APPARATUS OF RETRIEVAL OF CODED INFORMATION FROM SYMBOLS HAVING CODED INKS HAVING PHOTOLUMINESCENT COMPONENTS WITH SHORT AND LONG TIME CONSTANTS OF DECAY AFTER SHORT WAVE ILLUMINATION Filed Feb. 9, 1966 3 Sheets-Sheet 5 I 4-05 TE C TOR PROCESS/N6 ,--/6

CIRCUITS MERCURY ARC LAMP- I5 INVENTOR. FREDERICK HALVERSOIV BY fl a/Li l I ATTORNEY United States Patent 3 412,245 METHOD AND APPARATUS 0F RETRIEVAL OF CODED INFORMATION FROM SYMBOLS HAV- ING CODED INKS HAVING PHOTOLUMINES- CENT COMPONENTS WITH SHORT AND LONG TIME CONSTANTS OF DECAY AFTER SHORT WAVE ILLUMINATION Frederick Halverson, Stamford, Conn., assignor to American Cyanamid Company, Stamford, Conn.,

a corporation of Maine Filed Feb. 9, 1966, Ser. No. 526,184 6 Claims. (Cl. 250-41) ABSTRACT OF THE DISCLOSURE Coded information in which the code is based on the presence or absence of photoluminescent constituents in an ink, which constitutents have different rates of decay of photoluminescent radiation after excitation with ultraviolet light. The coded symbols are illuminated with ultraviolet light, which may be steady or pulsed but should have a definite repetition rate even if steady, i.e., an AC component. Photoluminescence is received by a detector and the signal is processed in processing circuits which separate DC from AC. When the reciprocal of the time constant or period to decay to a certain value after irradiation is very large compared to the AC component in the illumination, the luminescence mimics the excitation. On the other hand, when it is small compared to the AC component, the luminescent output is of approximately constant intensity, with only a small AC ripple. The ratios of AC to DC components vary greatly with the decay time, and the signal from the processing circuits determine the presence or absence of the different components. When pulsed radiation is used with filters, a synchronous time sequence can be produced, for example with a disc having various slots. The short time constant components will produce photoluminescence which is detected only by the first slot, whereas the longer time constant ones will have luminescence distributed over more than one slot. In each case the distinction is on the basis of time constant of decay or its reciprocal rather than color of the photoluminescent lights, in an extreme case even if the components all had the same color but had very different rates of time decay. Advantageously some spectral separation of the luminescent lights is provided, which decreases the overlap in luminescent wavelengths and permits sharper, more reliable reading of coded inks.

Related applications A general application on coded inks with various photoluminescent materials is described and claimed in the co-pending application of Freeman and Halverson, Ser. No. 596,366, filed Oct. 14, 1966, which was a continuation-in-part of an earlier application, Ser. No. 437,866, filed Mar. 8, 1965, and now abandoned. Both applications were assigned'to the assignee of this application.

Background of the invention In the application of Freeman and Halverson above referred to, there is described a method and apparatus for recording and retrieving information by means of photoluminescent materials. In the application so-called coded inks are utilized in which various symbols, such as numbers, are represented by the presence or absence of particular photoluminescent material rather than representing the symbols by particular shapes which are distinguishable either visually, magnetically or by other r: CC

characteristics. When dealing with numbers, for example, four different photoluminescent materials may be used, and this gives the possibility of fifteen different codes, the general formula being 2 1, where x is the number of photoluminescent materials. Larger numbers of coded materials permit representing still larger numbers of different symbols. For example, with six different photoluminescent materials sixty-three symbols are distinguishable. The different coded inks contained the necessary mixtures of materials which fluoresced under ultraviolet illumination in definite colors.

The code was read by illuminating with ultraviolet light and detecting presence or absence of a particular luminescent material by individual radiation detectors provided with filters or other spectral separation means so that each detector responded only to the range of Wavelengths corresponding to the particular substance present. Among the photoluminescent materials preferred in the application referred to above were complexes of rare earth metals having an atomic number greater than 57, which are usually referred to as lanthanide ions. The complexes, such as chelates, were formed with various organic ligands, which under ultraviolet illumination excite the lanthanide ion into a metastable electronic state from which the chelated lanthanide ion can emit a photon having an energy corresponding to transition to a lower electronic level of the chelated ion. This emission is over a very narrow range of Wavelengths.

There was also described in the application referred to above the use of an ordinary fluorescent material such as diphenylanthracene. However, usually it is only possible to have one such ordinary fluorescent material because of the breadth of the wavelength range of fluorescence. Diphenylanthracene, for example, fluoresces with a peak in the blue, but some of the energy extends on into the green and violet. A somewhat better organic fluorescer is 4,5-diphenylimidazolone-Z, which has a somewhat better wavelength band of fluorescence and responds to the same ultraviolet wavelengths as the lanthanide chelates. Even with the very sharp, narrow fluorescent radiation of the chelated lanthanide ions, problems arose because the wavelength ranges sometimes were too close to each other and some overlapping took place and, of course, the problem was even more serious and more complicated with ordinary fluorescent substances such as diphenylanthracene, where the range of wavelengths of radiation is much larger.

Summary of the invention The present invention is directed to an improvement in the information retrieval from coded inks such as is described in the application of Freeman and Halverson above referred to. The invention provides an electrical method which produces sharp separation of signals based not only, or not even primarily, on the wavelength range of the photoluminescence but on the relative rates of decay of the radiation after excitation with ultraviolet or other shortwave radiation. One may consider that the signals produced by the method and apparatus of the Freeman and Halverson application had a certain signalto-noise ratio, that is to say, sharpness and reliability with which the signal denoting presence or absence of a given component can be distinguished from other signals produced which may be regarded as noise. The present invention can be considered in one aspect as providing a greatly improved signal-to-noise ratio, and it permits not only a sharper or more accurate reading of coded ink symbols but also permits the use of more than one subtance having a broader fluorescent spectrum. In other words, it reduces the uncertainty produced by overlap of the radiations from the different photoluminescent materials,

3 which overlap may be considered in the final output signals as a kind of noise or spurious signal.

Different photoluminescent materials emit radiation which decays at very different rates after illumination with shortwave radiation, which for the remainder of this specification will ordinarily be referred to as ultraviolet illumination as this is the most common one used, it being understood however that in certain cases shortwave light, such as blue or violet light, or even shorter wave radiation than ultraviolet may be used. Since the exact wavelength of the exciting radiation forms no part of the present invention, the remainder of the specification will describe the invention in conjunction with instances where the excitation of photoluminescent material will be by ultraviolet light.

The lifetime or time constant of photoluminescence is ordinarily referred to as that time within which, in the absence of continuing excitation the intensity of photoluminescence has decreased to a value 1/e of the initial intensity. It is common to symbolize these time constants by a letter, and in the present specification the letter '7' will be used for this purpose. Also, as will appear below, the reciprocal of 1- will often be referred to and is symbolized by k. Various photoluminescent materials have Ts which will range from as low as sec. (diphenylanthracene has a 7 of the order of magnitude of 10 sec.), up to Ts in the milliseconds or substantial fractions thereof. Certain inorganic photoluminescent materials are known which have even much longer Ts running into the seconds or even minutes or hours. However, the most common photoluminescent materials for the purposes of the present invention are either organic fluorescent materials, such as diphenylanthracene, with very short Ts, or lanthanide complexes where the Ts are of the order of a millisecond or sizable fractions thereof. The present invention distinguishes between different photoluminescent materials in terms of 1-, or more commonly k. It will be apparent that primarily this distinction is not based on sharply separated, very narrow photoluminescent radiation ranges for the different materials, and at least theoretically it would be possible to use broad band photoluminescent materials with any given amount of overlap as far as range of radiation wavelength is concerned. It is, however, desirable and included in a specific modification of the invention to reinforce the discrimination on the basis of k with spectral discrimination on the wavelength of the light. This increases the sharpness or, looking at it another way, makes for even better signal-to-noise ratios and so for some purposes is preferable.

Readout illumination with ultraviolet light, in general, involves two types of illuminators, namely a so-called steady state illuminator, such as a mercury vapor are or AC modulated noble gas discharge tube, and a pulsed illuminator, such as flashing discharge tubes, for example xenon tubes. The former may have a profile of intensity versus time ranging from a straight line parallel to the time axis, to a complicated, although periodic waveform. The latter results in definitely pulsed illumination and lends itself particularly to a combination of spectral separation with the separation on the basis of k which is the essential feature of the present invention. This also lends itself to a particular apparatus organization which is included in another aspect of the present invention.

The power source for an illuminator provides current appropriate to the lamp and the conditions desired. In connection with the present invention illuminators providing a time dependent intensity for excitation of luminescence are desired. While it is possible to operate an illuminator with a steady current to obtain a constant output intensity of radiation and then modulate this intensity with time by some external device, such as a chopper, before using it to excite luminescence, usually a more convenient method is to develop the time dependence in the illuminator output by providing a time dependent current flow to the lamp. This time dependent current may range from modulated direct current, such as an AC component superimposed on the DC component with an amplitude not greater than the DC com ponent, to a situation where the net current fiow actually changes direction with time, such as alternating current. As an example of the former situation, certain high intensity xenon discharges are operated in this manner, with some direct current flowing at all times to improve stability of the discharge. As an example of the latter situation, mercury arcs frequently are operated with AC supplies.

Since the current supplied to the lamps in both situations under practical conditions exhibits a time variation which is essentially repetitive, it may be referred to as a periodic time dependent current. In the following it will be assumed that illuminators consist of mated lamps and power sources, that is, operable combinations. Xenon lamps operated as described above, AC operated mercury lamps, or other lamps operated such that a significant alternating component is present in the radiative output from the illuminator, whether of sinusoidal or other waveform, are classed as lamps fed by a periodic time dependent current.

Since the radiative output from this type of illuminator increases and decreases periodically, fluorescent components are subjected to periods of stronger and weaker excitation. The frequency of this alternation is referred to as an excitation repetition rate.

The operation of the present invention, even in the worst case situation where the luminescence is of the same, or nearly the same, wavelength, can be made clear by the following mathematical illustrations, which will first be developed in connection with steady state ultraviolet illuminators. Consider a lamp intensity exhibiting a sinusoidal waveform, such as a DC xenon lamp with a superimposed sinusoidal AC modulation. The intensity, I(t), can be represented mathematically by the expression where A is the average value of I(t), a is the fractional amplitude of modulation (agl), and w is related to the frequency of modulation, by the expression w=21rf. For the materials of primary interest in the present invention, the rate at which a particular component is excited to its luminescent state can be described by the expression NI(t), where it is proportional to the absorption coefiicient for exciting radiation and N is the concentration of the component. The rate at which compo nents decay from the luminescent state is represented approximately by kn, Where It is the concentration of components in the luminescent state, and k has the meaning set out above.

The approximate net rate of change of excited molecules with time is described by the expression N/l(1+a cos wt)lcn The steady state solution of this equation is where 1 =k(k -f-w and =arc tan w/k. The intensity of luminescence is proportional to the concentration of components in the luminescent state, and so it is proportional to 11.

Looking at the above expression it will be seen that 1; is an attenuation factor for the modulation of the luminescence as compared to the modulation of the exciting radiation. When k w the attenuation factor is essentially unity and the modulation has its maximum value (lumi nescence mimics excitation). When k w the attenuation factor is small, thereby decreasing the extent of tmodulation and giving the the luminescence output the appearance of a constant intensity with a small ripple superimposed. The intensity signals, after being converted into electrical signals via phototubes or other radiation detectors, for components with widely differing ks are easily separated, as will be brought out below.

The invention will be illustrated simply with a two component ink, one component having a time constant of 100 ,usec. or k: secf and another component having a time constant of 1 msec. or k=10 S6Cf' For simplicity consider a light source modulated at 1000 cycles per second, w=21rf=6.28) 10 radians per second, with a modulation amplitude a=0.8. For the first component the attenuation factor is 0.85 while for the second it is 0.16. Hence the modulation present in the luminescence from the first component is 0.68, and from the second component it is 0.13.

Now let us assume the two components luminesce at the same or very nearly the same wavelengths. The luminescence is received by a radiation detector and an electrical signal is produced which essentially is made up of four parts:

where K is the maximum value of I(t) and sin wt/ is the absolute value of sin wt. This function can be expressed in terms of the infinite series 3 cos (iwt- Proceeding as before 1+2 3 cos 2wt2/15 cos 4wt+ .7cn,

and the steady state solution has the form n= Z 1+2/3 cos (Zest-(p '-2/15114 COS (4cotq54) +2/35n cos (6wi4 where .,=arc tan and a=2, 4, 6,

it Again there is a marked diflerence in the time dependence of the intensity of luminescence when k 2w and when k 2w. In the former case there is a large constant component of luminescence with a small ripple, this ripple consisting largely of the second harmonic of the frequency used to excite the lamp. In the latter case the constant component is negligible and the intensity follows quite closely the time dependence of the input. As before the electrical signals from a phototube monitoring the luminescence from components with widely differing ks are easily separated.

With a rectifying steady state illuminator, (mercury are), operating at 400 cycles per second, the invention will be illustrated simply with a two component ink, one component having a time constant of 10* sec. (or k: 10 SGCF'I) and another having a time constant of l msec. (or k=l0 sec- Since f=400 cycles per second, 4 1rf= 5024 radians secand the luminescene intensity from the first component is largely a fluctuating one which mimics the exciting intensity, while the luminescene intensity from the second component will have a large constant component with a small, essentially 800 cycle, ripple. (For the first component 712=1]4=77 1, while for the second component =0.19, m=0 .099, n =0.033.) The luminescence is received by a radiation detector and an electrical signal is produced which essentially is made up of only three parts:

Brief description 0] the drawings FIGURE 1 is a diagram in partial schematic of the electrical circuits for steady state illumination readout;

FIGURE 2 is a diagrammatic representation of apparatus for pulsed illumination;

FIGURE 3 is a diagrammatic representation of apparatus for steady state illumination.

Description of the preferred embodiments FIGURE 3 is a diagrammatic illustration of a continuous illuminator. The coded ink symbols are shown on a substrate at 8 with an illuminator 15, which may be a mercury arc or a modulated xenon tube. The illumination passes through a base 17 which is transparent to ultraviolent light and illuminates the coded symbols. They luminesce, and the visible light of luminescence, or in some cases very near infrared luminescence, is imaged on a photo tube 14, the output of which is passed into processin g circuits 16.

The processing circuit and the signals received are shown in FIGURE 1 Where it will be seen that the electrical signal from the two components, as described above for a sinusoidally modulated xenon discharge tube, encounters a frequency filter with a low pass filter made up of an inductance 1 and a large capacitor 2, and a high pass filter with a smaller capacitor 3, a rectifier 4, and a conventional smoothing filter made up of a resistance 5 and a capacitor 6. The values of inductance 1 and capacitor 2 are such that practically no AC components pass, but only the steady state DC from components #1 and #2. The signal leaving the filter of 5 and 6 represents the rectified and smoothed signal arising from the fluctuating or AC portion of the luminescence intensities of components #1 and #2. Representing the electrical signals generated by the two components in the photodetector monitoring the luminescence by the photodetector monitoring the luminescence by B [1+/3 cos (wt)] and B [1+;3 cos (wt(p the composite signal has the form (Pr-WH COS (wt0) Where W 0 a1c tan B151 cos +3 5 cos we The DC portion is 1B1)( 2. 2) 00S (1*2)l and the AC portion has the amplitude Under normal conditions of usage three sets of conditions have relevance, namely B B B B and B --B all with k k The instrument can be calibrated using ordinary DC meters and illuminating an ink which has only component #1, an ink which has only component #2, and an ink which contains both components, and then taking the ratios of the AC to the DC signals. For the particular combination of components cited previously in connection with a xenon lamp subjected to 80% modulation at 1000 cycles per second 20.68, fi :0.13, :l.4l5 radians, :0.56 radians), the ratios are -2 for B B -0.2 for B B and -0.63 for B 213 none of them being zero. These ratios are well separated and easily distinguished without need for separation of the wavelengths of luminescence. The other condition which can occur, namely, B =B =0, is recognized by the lack of either an AC or a DC signal.

The discrimination, or signal-to-noise ratio, can, of course, be further improved by any spectral separation which may be present. For example, if component #1 is a samarium complex with k of about sec. and component #2 is a europium complex with k of about 10 S6C."1, broad band optical filter arrangement can allow luminescence in the range 5750 A.6550 A. to impinge on the photodetector. Luminescence from a samarium complex in this range consists primarily of two bands at about 5980 A. and 6420 A., and from a europium complex consists primarily of a band at about 6120 A. The simple broad band filter arrangement would permit both Samarium luminescence bands to contribute to the detection, while the different time dependence of the two luminescent components allows differentiation between their presence or absence. Instead of the broad band filter and one detector, two detectors might be used, one fitted with a narrow band filter centered at about 5980 A. and the other with a filter centered at about 6120 A. Even in this case there can be a slight overlapping of the samarium luminescence band at 5980 A. and the europium luminescence band at 5980 A. and the europium luminescence band at 6120 A., depending on the particular complex. The presence of the narrow band filters, however, does not allow the condition B =B except where both are zero, and so the only ratios observable from the detectors are -2 or -0.2.

FIG. 1 applies equally well to the situation described with illumination by a rectifying lamp, such as a mercury arc. Taking a the first luminescent component diphenylanthracene with k about 10 secf and as the second component a terbium complex with k about 10 S6C. 1, and considering a lamp operated with a 400 cycle current supply, the first component makes a negligible contribution to the DC. signal from the photodetector, that is V(DC, #1):0. As before, the instrument can be calibrated using ordinary DC meters by illuminating an ink which has only component #1, then illuminating an ink which has only component #2, and finally illuminating an ink which contains both components. The signal V(DC, #2) is a direct measure of the presence of component #2, the ratio of the rectified AC signal to V(DC #2) determines the presence or absence of component #1. In fact, using a set of known mixtures of com ponents #1 and #2 in varying ratios, a continuous calibration curve can be developed which is valid as long as some of component #2 is present. The presence of an AC signal in the substantial absence of a DC signal is a clear indication that only component #1 is present.

It should be noted in the illustration used above that contributions of higher harmonics to the AC portion drop off rapidly for component #2, but very slowly for component #1. It is practical to construct more complex electrical filter assemblies, such as band pass filters, which will separate different frequencies more sharply, and so the higher harmonic content can be used as a further means for discrimination between luminescent components. This becomes more important when more than two luminescent components are involved.

The steady state illumination methods described in connection with FIG. 1 were illustrated using a modulation frequency of 1000 cycle in the case of the xenon discharge, or a 400 cycle current source for the mercury arc. Other frequencies may be used, and it is an advantage of the invention that the best frequency for any particular purpose can be chosen. The choice of frequencies can depend on the time constants of the luminescent materials and it is also possible with suitable electrical filters to operate the system with a frequency scanning device which can select in sequence a number of frequencies. Such a more elaborate modification can be used wherever the nature of the symbols make it worth its additional complexity. A simple, if somewhat crud-e, illustration will suflice. Let use assume three photoluminescent materials, one having a time constant of about 10 and another one 10 and a third 10" If a single frequency is used, for example 1000 cycles, there will be a sharp separation between signals from the component with 10 time constant, but the other two will be more or less lumped together. If, however, scanning or other time sharing circuits are used, one frequency being 1000 cycles, the next 10 kc., and the third kc., there will be a sharp separation also between the components with the time constants of l0- and 10 FIG. 2 shows a different modification in which the exciting radiation, instead of being continuous AC, is pulsed, for example, a xenon flash tube 7 operating in a circuit which causes it to flash upon receipt of a signal, the luminescence from the coded ink in location 8 having the components to be determined is then spectrally separated, for example by a prism or grating which disperses it spatially, FIGURE 2 illustrating this dispersion by means of the prism 9. A rotating disc or wheel 10 is synchronized with the xenon flash and is provided with a series of slits 11 in its periphery. A single radiation detector, symbolized by a photo tube 12, receives the radiation coming through the slits. The pulsing of lamp 7 is synchronized with the rotating disc or wheel by means of light from source 13 generating a signal from phototube 14 each time a slit 11 passes in front of it, causing lamp 7 to flash once for every second or higher signal from phototube 14. The particular sequence of flashes is determined by the electronic circuits in the lamp flash supply. These circuits are not shown as they are conven tional, for example an ordinary flip-flop for flashing on every second signal. The location of the source 13 and phototube 14 combination around the periphery of the disc is adjustable to provide appropriate synchronization. Let us assume a positioning of slits and rotation of disc such that the slits sample radiation at 500 sec. intervals and they scan through a spectrum, i.e., blue, green and red, in about 200 ,usec. Assume the flash duration is about 200 ,asec. If now we have two components, diphenylanthracene with a time constant of about 10- sec. and a terbium complex with a time constant of about 1 msec., the diphenylanthracene will be perceived only through the slit which synchronizes with the xenon flash. The next slit some 500 ,usec. later will receive no energy from the diphenylanthracene. The slit which synchronizes with the green radiation from the prism, representing, let us say, the luminescence from terbium, will give out a signal and so will the next slit because the lifetime of the terbium complex is of the order of 1 msec. or a little less. In other words, there would be response from terbium for two slits whereas the green portion of the diphenylanthracene luminescene would only come through a single slit. Therefore, the overlap would cause no particular problem because if a time dependent readout is used, such as for example an oscilloscope, terbium would be represented by two lines or signals on the oscilloscope whereas diphenylanthracene would be represented only at the point corresponding to the blue. The green response from diphenylanthracene would show up as only a single line at the time of actual flash because of the extremely short lifetime of the luminescence from the diphenylanthracene.

A prism or other means for separating the different spectral regions of luminescence in terms of physical displacement is illustrated in FIG. 2 and is a very satisfactory form. However, a similar result can also be obtained without spatial dispersion. For example, focus an image of the luminescing area on the surface of the rotating disc at a radial distance from the center of the disc corresponding to the slits. Replace each slit, and the area along the periphery adjacent to each slit, with a set of optical filters corresponding to the wavelengths of luminescence to be detected. Each filter can have the dimensions of one slit, and the sequence of filters in a set must be the same for all sets. It the luminescent components of interest are diphenylanthracene and a terbium complex, the slits can be replaced by a narrow band interference filter transmiting the 5430 A. terbium and a similarly shaped blue filter can be inserted in the .disc at the same radial distance but displaced by a small angle from the green filter. The blue and green filters can be adjacent or slightly separated, but the arrangement must be the same at each slit location. If desired, a stationary slit may be placed in front of the photodetector such that when a given filter is in front of the slit, only the radiation passing through that filter reaches the detector. This reduces the amount of stray light. As the disc rotates, the signal from the detector has the same characteristics as previously.

Another modification involves retaining the slits as initially described, but diverting the luminescence after passing through the slits to several photodetectors, each one equiped with an appropriate filter. For this configuration the outputs of the separate detectors represent luminescence at different wavelengths, and they are sampled simultaneously instead of sequentially as previously. Since the flashtube is pulsed only for alternate slits, alternate signals represent a fixed time delay after excitation, say 300 sec.

In each of the above methods there will be a clear distinction between radiation from photoluminescent materials with very short time constants and materials having longer time constants, even though there is spectral overlapping of luminescence bands.

Thepulse method may also be used with a fixed slit in front of the detector across which the spectrum is swept, for example by means of a rotating or oscillating mirror.

While the descriptions of pulsed operation have been in terms of electrical pulsing of the flashtube followed by mechanical time discrimination, all of the operations can be performed electrically also. For example, if a prism or other means of spectrally dispersing the luminescence is used in the manner shown in FIG. 2, stationary slits can be located spatially so as to pass the desired luminescence bands. Individual photodetectors are mounted behind each slit, and the detectors are connected into circuits I which permit sampling the individual detector outputs at selected time intervals during and subsequent to a flash. The flashtube is connected into a circuit which drives it at a fixed repetition rate, such as 60 cycles. If no spectral dispersion is used, the fluorescent radiation can be directed to a multiplicity of detectors equipped with appropriate filters, and time sampling of detector outputs handled in the same fashion.

I claim:

.1. A method of indicating presence of photoluminescent material in a coded symbol which comprises:

(a) illuminating the symbol with shortwave radiation of periodic time dependent intensity,

(b) detecting luminescence from the symbol and transforming luminescence intensity into electrical signal, whereby luminescent components having reciprocal luminescence lifetimes of the order of magnitude of the excitation repetition rate contribute a signal having a large DC component with a small AC ripple and luminescence from components having reciprocal lifetimes very much greater than the excitation rate contribute to the output signal AC components with a substantially smaller DC component,

(c) separating the signal into components by frequency including at least one low passfilter whereby luminescence from components having relatively long time constants contribute a DC component to the output and further discriminating against low frequency components to produce a signal having AC components corresponding to luminescence of short time constant and a relatively smaller alternating current signal corresponding to the ripple produced by the luminescence of relatively long time constant components, and

(d) producing a ratio of AC to DC.

2. A method according to claim 1 in which the AC signals are rectified and indicated on DC responsive instruments.

3. A method according to claim 1 in which at least one component with relatively longer time constant of luminescence is a chelate of a lanthanide ion.

4. A method according to claim 2 in which at least one component with relatively longer time constant of luminescence is a chelate of a lanthanide ion.

5. An aparatus for determining the presence of components in a coded symbol, which components have photoluminesce of widely different luminescence time constants which comprises in combination:

(a) means for illuminating a substrate carrying a symbol with pulsed shortwave illumination,

(b) means for separating the luminescent radiation spectrally,

(c) means for detecting the different spectral radiations in timed sequence after spectral separation, and

(d) means for synchronizing the timed sequence with the pulse rate.

6. An apparatus according to claim 5 in which the means for detecting the different spectral radiations and for synchronizing the timed sequence thereof which pulse -rate comprises:

(a) a radiation detector and a disc provided with slits,

and

(b) means for driving the disc in synchronism with pulse rate, the disc and slits being oriented to pass beams spectrally dispersed by the dispersing means to the radiation detector.

References Cited UNITED STATES PATENTS 3,334,235 8/1967 Zarowin 2507l RALPH G. NILSON, Primary Examiner.

M. I. FROME, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,412,245 November 19, 1968 Frederick Halverson It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 47, "it" should read p Column 6, lines 9 to 13, the portion of the equation reading "l/(AC, should read V(AC, #l)+ lines 49 and 50, cancel "the photodetector monitoring the luminescence by"; line 51, "(wt-q)" should read (wt-4 line 55 (22)" should read (p lines 58 to 60 the portion of the equation reading "B +B should read B 8 line 66, "(B (B should read (B 6 Column 7, line 71, "cycle" should read cycles Column 9, line 25, "equiped" should read equipped Column 10, line 32, "aparatus" should read apparatus line 46, "which" should read with Signed and sealed this 31st day of March 1970.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR Attesting Officer Commissioner of Patents 

